Sentence Fragments—Tricky Little Devils

In formal writing those unfinished bits that lack a subject and often a verb are studiously avoided. But they have a real place in fiction and informal writing. There sentence fragments can be used to great effect or they can bomb, jarring the reader right out of the story. How to use them is hard to explain, at least for me. My ear tells me when they fit, but I still can't always say why.